The Orthogonal Frequency Division Multiple Access (OFDMA) communication method is adopted in mobile WiMAX as one of the standards for next-generation high-speed data communications or in the downlink of Long Term Evolution (LTE). In OFDMA, radio resources are two-dimensionally allocated, in a time direction (symbol direction) and a frequency direction (subcarrier direction). In the OFDMA communication method, a frame is segmented into a plurality of zones in the time direction, and different subcarriers are arranged for the zones (in distributed subcarrier allocation or adjacent subcarrier allocation).
A structure of such a frame is illustrated in FIG. 12. A downlink subframe of the frame illustrated in FIG. 12 includes a preamble signal, a zone Z1 with a distributed subcarrier applied thereto, a zone Z2 with an adjacent subcarrier applied thereto, and a zone Z3 with an adjacent subcarrier applied thereto. A transmit transition gap (TTG) is a guard period within which a radio base station (hereinafter simply referred to as base station) switches from transmission to reception, and a receive transition gap (RTG) is a guard period within which the base station switches from reception to transmission.
Zone switching is performed in order to selectively use broadcast communication to all users or communication for controlling transmission in directivity to a particular user. As illustrated in FIG. 12, the broadcast communication is allocated to the zones Z1 and Z2, and the communication for the particular user is allocated to the zone Z3.
In addition to user data (user symbol), a reference signal, such as a pilot signal (pilot symbol), is exchanged between a base station and a mobile station in order to estimate a channel. Different pilot allocations (pilot subcarrier allocations) are used for respective subcarrier allocations so that a channel estimation accuracy and a band usage efficiency are increased.
FIGS. 13 and 14 illustrate different pilot allocations. A pilot allocation PC1 illustrated in FIG. 13 is used for distributed subcarrier allocation, and a pilot allocation PC2 illustrated in FIG. 14 is used for an adjacent subcarrier allocation. As illustrated in FIG. 13, odd symbols and even symbols of the pilot symbols are arranged in a shift of several subcarriers in the pilot allocation PC1. As illustrated in FIG. 14, the pilot symbols are allocated to particular subcarriers in the pilot allocation PC2.
The mobile station receiving the frame compares a pilot symbol received from the base station with a known pilot symbol, thereby calculating an estimated value of a channel from the base station to the mobile station (downlink). A channel estimated value related to a symbol with no pilot symbol allocation (for example, a data symbol) is determined by linearly interpolating in a time direction and/or a frequency direction the channel estimated values, calculated based on the pilot symbol.
As illustrated in FIG. 13, the channel estimated value at data symbol D22 is determined as the mean value of the channel estimated values of pilot symbols P11 and P31. The channel estimated value of data symbol D21 is calculated as a linear interpolation value between the channel estimated values of a pilot symbol P21 and the data symbol D22.
Referring to FIG. 14, the channel estimated value at a data symbol D61 is calculated as a linear interpolation value of the channel estimated values at pilot symbols P61 and P62. A calculation accuracy of the interpolation value is increased by using pilot symbols P51, P52, P71, and P72 consecutive in the time direction.
Generally, the channel estimated accuracy (accuracy of the interpolation) is increased by inserting a large number of pilot symbols in the frame. However, the number of data symbols inserted is accordingly reduced, and data transmission efficiency (throughput) is thus reduced. Increasing the data transmission efficiency limits the number of pilot symbols that can be inserted into the frame in accordance with the pilot allocation.
Japanese Laid Open Patent Publication 2007-150971 discloses a radio communication method that allows the number of pilots to be variable within a frame addressed to a mobile station in response to reception quality.
A zone boundary between adjacent zones where different pilot allocations are applied in a frame is now considered. In such a zone boundary, positional continuity is lost in the time direction with the pilots allocated, as illustrated in FIG. 15, and the accuracy level of the channel estimation may be lowered. In particular, if there is a large variation in a propagation channel, a decrease in the channel estimation accuracy becomes pronounced.
The adjacent zones may have the same pilot allocation, but may be different in pilot directivity. In such a case, neither an interpolation operation nor averaging operation can be performed in the time direction using the pilots of the adjacent zones in the vicinity of the boundary of a schedule target zone. Thus, the channel estimation accuracy is lowered. In one case of different directivities of the pilots, a downlink communication is performed in a beam forming operation to a particular mobile station allocated to a schedule target zone. In such a case, the propagation channel is different from that for broadcast communication. Thus, even if the adjacent zones have the same pilot allocation, neither an interpolation operation nor averaging operation can be performed in the time direction using the pilots of the adjacent zones in the vicinity of the boundary of the schedule target zone.
An edge region close to the edge of a zone in the frequency direction is considered (see FIG. 16). Continuity of the positions of the pilots is naturally lost in the frequency direction in the edge area. Thus; the channel estimation accuracy may be lowered. In particular, if there is a large variation in a propagation channel in the frequency direction, the decrease in the channel estimation accuracy becomes pronounced. The decrease is particularly pronounced if the adjacent subcarrier allocation, which is not expected to provide frequency diversity advantage, is applied.